Forum » Engine Building Fundamentals » Alignment


Engine Building Fundamentals

Discussion and questions related to the course Engine Building Fundamentals

= Resolved threads

Page 1

My question is why does the cylinder need to be centered with the cranks center on a inline engine because since it rotates in one direction why can't the cylinder be moved slightly in front of the rotational motion?

It has been done. This was/is done to reduce the side loading on the major thrust face and improve the piston/con'rod angle of the force on the crankpin, it increases the side loadings on the minor thrust face but that is relatively minor. IIRC, this practice is specifically prohibited in F1 and many other race series, so it would seem that there is potential. However, moving the crank is a lot of work to get everything working together so you are really limited to moving the bore and this is going to be limited by the cylinder wall thickness, unless you have the insert seats (?) offset machined, and there may be other issues with specially made head gaskets and moving the head over if required.

It is common practice for manufacturers to offset the gudgeon/piston/wrist pin a little to quieten it as it rocks around TDC - piston slap - and an old-school trick was to reverse the pistons to get the equivalent effect of a better rod angle at the cost of more piston noise... don't recall how effective it was, though.

You may wish to look at some of the very heavy diesel and steam engines used in shipping as, to remove the drag from the piston's side thrust on the cylinder and/or to allow very long stroke to bore ratios, they use a form of 2 piece connecting rod with the centre joint/knuckle sliding in guide to prevent lateral movement. Sometimes the upper section is bolted directly to the piston to help stabilise it, sometimes a pin is used. Don't know if they're also offset, but if there is an advantage in it, it will be used.